چکیده انگلیسی

Down syndrome (DS) is associated with a specific verbal short-term memory (STM) deficit. This study explored the effects of grouping, semantic relations and visual presentation upon verbal STM recall performance in a group of 15 individuals with DS and 15 vocabulary-matched typically developing (TD) children. Participants were presented with memoranda in either a temporally grouped schedule, such that items were grouped as pairs, or in an equally spaced presentation schedule. The two items constituting each pair were either semantically related or unrelated. Performance across these conditions was compared in verbal or verbal plus visual presentation modes. Significant memory recall benefits were observed across populations as a result of temporal grouping, semantic relations and verbal and visual combined presentation. However, a reduced benefit of semantic relation in the DS group compared to the TD group indicated that those with DS were less influenced by LTM relational knowledge. In addition, those with DS only experienced a grouping benefit during verbal and visual combined presentation, in contrast to the TD group who experienced grouping benefits throughout. This indicates that individuals with DS are poorer at encoding temporal context for purely verbal memoranda. These findings were replicated in a follow-up experiment, aimed at aligning baseline performance in the two populations. This study provides encouraging evidence that, despite their difficulties in some areas, individuals with DS can benefit from the use of grouping and LTM knowledge to assist their verbal STM performance under certain circumstances.

مقدمه انگلیسی

Verbal short-term memory (STM) is responsible for the short-term storage and maintenance of verbal input such as words and digits. Associations between individuals’ verbal STM performance and the development of vocabulary acquisition, comprehension and syntax are well established (Baddeley et al., 1998, Baddeley et al., 1988, Ellis and Sinclair, 1996 and Vallar and Baddeley, 1984). In turn, this has driven research seeking training routes to enhance memory performance. Recent memory training programmes have primarily aimed at improving the broader concept of working memory, e.g., CogMed (www.cogmed.com), that is, individuals’ ability to store and simultaneously process information, be it verbal or visual. The relevance of such work follows from the fact that working memory is implicated in various wider abilities such as mathematics and reading ability (Gathercole et al., 2004 and Leather and Henry, 1994). Nevertheless, given the role of verbal STM in driving aspects of language development, understanding how one might improve verbal STM performance specifically is also extremely relevant for populations known to experience particular problems in the language domain.
One population among whom verbal STM tends to be specifically poor is those with Down syndrome (DS). Individuals with DS experience varying degrees of general learning difficulties. Nonetheless, verbal STM is a domain in which, as a group, specific difficulty is consistently displayed, with performance on verbal STM tasks being significantly poorer to that observed in comparable visual STM tasks, as well as significantly poorer than the performance of matched control groups, be they typically developing, or individuals with other learning difficulties (Brock and Jarrold, 2005, Hulme and Mackenzie, 1992, Jarrold and Baddeley, 1997, Jarrold et al., 2000, Jarrold et al., 2004, Laws and Bishop, 2003 and Vicari et al., 2004).
Verbal STM has a limited capacity (Cowan, 2001 and Miller, 1956), according to Cowan (2010) this capacity is limited to around 3–5 chunks in typical adults. However, the amount individuals are able to recall during STM tasks can also vary according to other factors regarding the given recall circumstances, one being the structure or organisation of the items presented for recall. For example, it has long been known that presenting memoranda with grouping imposed on the items using perceptual separation, such as by separating items by temporal pauses, results in benefits to recall in typically developing groups. Such perceptual grouping effects are observed in both adults (Bower and Winzenz, 1969, Frankish, 1995 and Maybery et al., 2002; Melkman et al., 1981 and Ryan, 1969), and children (Harris & Burke, 1972), including pre-schoolers (Calfee, 1969). It has been suggested by Farrell (2012) that clusters (or groups) provide a hierarchical temporal context; recalling the context consequently helps an individual to recall the items within that group context. In this sense, grouping divides the verbal sequence into separate sub-sequences, such that individuals can process coarse temporal context for each cluster and a finer temporal context for the items constituting each cluster. Such a hierarchical structure allows for organised encoding and retrieval of items. This notion is also compatible with working memory models such as Burgess and Hitch's (1992) connectionist model, which invokes the concept of a context timing signal (see also Burgess, 1995). In Burgess’ (1995) model the ‘context’ is defined using the metaphor of a moving window, with item nodes forming temporary associations with other items nodes in that same window (shared context). Hence, temporally grouping items at presentation may lead to enhancements in performance, as discussed above, as a result of structured, hierarchically organised encoding and recall of items.
Grouping may be a useful strategy for individuals with DS to adopt given that they experience verbal STM difficulties and appear to have a reduced verbal STM capacity (Purser & Jarrold, 2005). One possibility is that a lack of strategic grouping may be a contributing factor to the consistent finding of poor verbal STM performance in this population. Spontaneous grouping tendencies for memoranda (that are not readily grouped at presentation), do not appear to develop in typically developing children until they reach around 8 or 9 years of age (Towse, Hitch, & Skeates, 1999). It is possible that similarly, without instruction, individuals with DS may not develop spontaneous grouping strategies for unstructured verbal input, given their mental age. Thus, for young children (and possibly individuals with DS) who would otherwise struggle to spontaneously group verbal memoranda it may be particularly useful to provide grouped structure at input. As such, temporally structured presentation of input may induce organised encoding and recall. Previous studies have attempted to actively train individuals with DS to enhance their organisational memory strategies, observing subsequent benefits (Broadley and MacDonald, 1993 and Broadley et al., 1994), with skills maintained at a later date, but appearing to be gains only on the specifically trained tasks. While individuals with DS therefore appear to benefit from organisational instructions, to our awareness, no existing studies have explored the nature of possible grouping benefits in individuals with DS simply by presenting memoranda in a format that can be readily organised into perceptual groups.
The first aim of the current study was therefore to explore whether individuals with DS benefit from grouped presentation of memoranda. This was done by measuring recall performance for lists of items presented at a temporally grouped rate, such that pairs (groups) were presented with the temporal interval between each pair being longer than the interval occurring between the two items constituting the pair. Recall for such stimuli was compared to that for items presented within a non-temporally grouped (equal) schedule. A benefit of grouped presentation would indicate that individuals with DS spontaneously make use of the pre-grouped format of the memoranda, storing and retrieving the items in memory according to the temporally grouped organisation. If recall performance is enhanced for those with DS, then this would have implications for the presentation of materials in educational settings.
The effect of temporal grouping upon those with DS was therefore compared to that seen in a typically developing (TD) matched control group of children. If individuals’ encoding and recall potential reflects a developmental progression, then one might expect that individuals with DS will benefit from temporal grouping to a similar degree to that of a vocabulary matched control group of TD children. However, given the specific verbal STM problems experienced by those with DS, whereby they tend to perform poorer on tests of verbal serial recall than matched controls, it may be that the temporally grouped presentation does not enhance recall for those with DS to the extent expected for their mental age. For example, the specific verbal STM deficit observed in those with DS may be due to differential processing of the verbal input, such that temporal structure is processed or encoded in a way that is not equivalent to their TD peers.
Another factor that supports verbal STM span performance in TD groups is the top-down influence of existing knowledge of the memoranda held in long-term memory (LTM). For instance, in immediate serial recall tasks concrete words are recalled better than abstract words (Paivio, Clark, & Khan, 1988), and high frequency words better than low frequency words (Roodenrys, Hulme, Lethbridge, Hinton, & Nimmo, 2002). Typically, individuals are able to remember more items during STM tasks when the items are meaningful (Caza and Belleville, 1999, Graf and Schacter, 1989 and Poirier et al., 2011). For instance, the 3 letters ‘U, S, A’ are more memorable than the 3 letters ‘B, J, T’ (Bower, 1972), due to the meaning associated with the letters in the former list. Meaning (e.g., for the 3 letters FBI), allows for multiple items to be encoded as one whole unit, or chunk. Accordingly, Cowan et al. (2010) report that it is both the size of the chunks as well as the number of chunks stored and recalled that increases across development, resulting in improvements in memory performance. Thus, one key strategy that individuals can use to increase memory span is to store verbal input in semantically organised chunks. Items constituting a semantic chunk will therefore have associations with one another, in a similar vein, some researchers also argue that effects such as frequency are, at least partially, due to item associative benefits (Stuart & Hulme, 2000), whereby items that co-occur more commonly develop associations with each other, thereby enhancing recall for these items. When items have semantic inter-associations they should therefore be easier to recall, whereby remembering one item then cues participants to also recall the associated item, perhaps with sets of related items constituting separate semantically organised chunks.
Thus, a second manipulation in this study was of the semantic relatedness of items. Specifically, items were presented in either a semantically related arrangement, such that two consecutive items were related, compared to when these same items were presented in a non-related arrangement (see Fig. 1). Each pair was first chosen on the basis of subjective judgements of relatedness; the two items in any given semantically related pair were considered by the experimenter to be clearly related, such as dog and bone, umbrella and rain, boat and sea. However, to provide an additional objective measure, latent semantic analysis was then used to confirm the degree of semantic relation for each of the chosen pairs of items, as further outlined below.
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Fig. 1.
Item arrangements across the 4 blocks, and temporal presentation durations (ms) for items when presented in the ungrouped schedule, and in the temporally grouped schedule.
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Although selective verbal STM problems are observed in DS, Jarrold, Baddeley, and Phillips (2007) found that a group of individuals with DS showed no specific verbal LTM deficit. Additionally, previous research indicates that those with DS are influenced by top-down processes such as word knowledge during STM recall. For instance, Brock and Jarrold (2004) found a significant lexicality effect in those with DS. This indicates that individuals with DS may also have the potential to benefit from semantic association manipulations during verbal STM tasks. Nash and Snowling (2008) found evidence that during a semantic fluency task the number of items in each semantic cluster was not decreased in their DS group compared to a matched TD group. However, individuals with DS did recall fewer clusters than the TD group. Nash and Snowling concluded that semantic organisation is not impaired in DS; however they suggest that their findings most likely reflect the presence of compromised retrieval strategies, such that the DS individuals are poorer at moving between clusters. Previous research therefore indicates that verbal LTM is organised similarly in those with DS and does influence STM performance, perhaps somewhat automatically, e.g., as shown by strong lexicality effects (Brock & Jarrold, 2004). However, strategic, organised retrieval based on semantic associations may be less efficient in those with DS compared to a TD comparison group (Nash & Snowling, 2008).
In the current study, for all trials, the memoranda were words presented auditorily. Auditory presentation was used in order to gain a relatively pure measure of verbal STM. However, it remains possible that individuals with DS rely on visual maintenance strategies to compensate for their difficulties in verbal STM tasks. For instance, rather than using existing verbal associations when presented with semantically related items, individuals might imagine the two items comprising a related pair in a single image. This type of interactive imagery was explored by Bower (1970). Bower found that when individuals were instructed to imagine two items interacting, they subsequently had better memory recall for these items, compared to when they used other strategies such as verbal rehearsal. In the current study, the addition of a picture for each item in the pair at presentation would be expected to encourage visual processing/visual strategies during maintenance and recall. If individuals experience larger benefits of semantic relation, or of grouping, during the stage when pictures are presented simultaneous to the verbal item (verbal plus visual presentation condition), then this would be an indicator that they are benefiting from a visual process, such as interactive imagery, or a visual grouping mechanism. The inclusion of a verbal plus visual presentation condition therefore had the potential to highlight key differences in strategy use in those with and without DS, which should increase our understanding of the verbal STM difficulties observed in this population.
Thus, to gain an understanding of the nature of potential grouping and semantic relation benefits, a visual presentation condition was also included whereby each relation × grouping block (see Fig. 1) was presented with a visual picture, in addition to the auditory label. The verbal only presentation condition was always carried out first to avoid encouraging participants’ use of imagery in the initial verbal only stage. As a result, participants may be expected to have slightly better overall performance in the visual stage.
To summarise, this study set out to test whether individuals with DS as well as matched TD individuals, experienced grouping benefits, encoding/recalling input according to the structured temporal context. Recall of items presented in a temporally grouped schedule, was thus compared to recall of these same items when presented at an equal pace. Second, this study explored whether those with DS, compared to those without, found it easier to recall items based on associations/semantically organised chunks, by recording participants’ recall of items that were presented in a semantically related arrangement, compared to when the same items were presented in a non-semantically related arrangement. This second manipulation was crossed with the first resulting in four temporal grouping × semantic relation blocks. Finally, performance across these blocks was compared when memoranda were presented verbally and when presented with a picture accompanying the verbal label. It was hypothesised that while the TD group would display better performance with the addition of visual presentation, those with DS may experience a larger benefit, since visual STM performance is comparably superior to verbal STM in individuals with DS. If the DS group do not experience the same benefits as the TD group as a result of the above manipulations, this may highlight areas that contribute to poorer verbal STM performance in those with DS, such as differential encoding of temporal structure, or poorer context based retrieval strategies, as well as possibly poorer strategic use of the semantic organisation of the input, or less activation of semantic relations in LTM.
In the current study, the aim was to explore whether individuals were better at recalling each unit (pair), as a result of grouping/relation. Parmentier, Andres, Elford, and Jones (2006) reported that for temporally grouped input, more within group item migration errors occurred, but with improvements in recall of items overall, in the correct group. Each pair may constitute a separate group or unit, as a result of grouping the two items, and/or due to associating two semantically related items in a given pair, potentially as semantically organised chunks. Given this, we scored items according to correct pair position, e.g., item ‘1′ and item ‘2′ of pair ‘a’ could be recalled correctly in either of the serial positions ‘1′ or ‘2′ of pair ‘a’; rather than strict serial recall (e.g., whereby item ‘1′ can only be recalled correctly in position ‘1′ of pair ‘a’). Thus, we allowed for the order of the two items of a given pair to be interchangeable.